The electronic radiofrequency identification systems (RFID, radio frequency identification device) are being increasingly widely applied.
In fact, these systems are already these days widely used for identifying, locating or tracking objects, animals and people (access cards; tolls; electronic passports, etc.).
They generally comprise a reader and a transponder, for example integrated in a support such as a tag or a card. In the present invention, reference is made to passive transponders, that is to say, transponders that do not have their own energy source for transmitting data.
The reader, considered as a base station, generates a carrier wave (short range alternating magnetic field) which is used to power the contactless cards in proximity to the antenna of the reader. This process is called remote power feed. Furthermore, this carrier wave provides the card with a clock for the operation of its various blocks. Lastly, it serves as a medium for the reader-card and card-reader communications.
A distinction is made in the exchange of information between the base station and the transponder between the channel from the reader to the card and the channel from the card to the reader.
For the latter, a retromodulation (of amplitude or of phase) is applied by the passive transponder to the carrier by controlling a variable load, for example resistive or capacitive, at the terminals of the antenna of the card.
This switching provokes an armature reaction on the side of the reader, which can be demodulated and decoded to obtain the information stored in the card.
Hereinbelow, the term “symbol” is used to mean an element of a data encoding alphabet. A symbol may, for example, be a set of 3 bits, such as 011.
In the present context, the term “pattern or modulation pattern” is used to mean a succession of load levels of a predefined length used for the physical encoding, that is to say the retromodulation.
A pattern corresponds to a symbol and is characterized by its length and a succession of load levels.
First of all, the standard ISO 14 443 should be cited, which defines and allows for a communication between a reader and a card to be set up on the basis of a 13.56 MHz carrier (fc) with a bit rate of between 106 kbits/s and 848 kbits/s. This standard proposes two retromodulation types (type A and type B), involving the notion of 848 kHz subcarrier (fSC=fc/16).
However, for certain applications, such as, for example, biometric identification or an electronic passport, this bit rate is not sufficient.
Various solutions have been proposed in order to increase the bit rate that can be achieved between the card and the reader.
A first approach proposed by the present Applicant consisted in increasing the frequency of the subcarrier to 1.7 Mbit/s (fSC=fc/8) and in modulating the phase of the subcarrier. Thus, bit rates of 1.7 Mbit/s and 3.4 Mbit/s were able to be achieved.
By moving away from the backdrop of the 848 kHz subcarrier frequency and by proposing a subcarrier frequency up to 27.12 MHz, other approaches have been able to achieve theoretical bit rates ranging up to 27.12 Mbits/s.
In more detail, a first obvious solution to this problem would consist in increasing the number of symbols.
The term “symbol” is used to mean the individual characters of an encoding alphabet. With a subcarrier from 848 kHz to 27.12 MHz and phase shifts Δφ of π, π/2, π/4, π/8, π/16, π/32, π/64 and π/128 makes it possible to cover a range of bit rates from 106 kbit/s to 27.12 Mbit/s.
A second obvious solution for increasing the bit rate with a constant number of symbols will consist in reducing the symbol time.
Thus, the symbols ‘0’ and ‘1’ are, for example, represented by a conventional Manchester code and the symbol time is variable. The bit rate is then equal to the subcarrier frequency.
In the case where the symbol time is equivalent to the period of the carrier at 13.56 MHz, there is therefore a bit rate of 13.56 Mbits/s.
However, it is observed that the bandwidth of the signals in base band increases very greatly with the bit rate, which hampers the recovery of the energy from the card and therefore embrittles its operation, and even reduces its range.
These two solutions which come naturally to the mind of those skilled in the art do not therefore address the problem posed.
Finally, a solution proposed for a UHF RFID application in the document WO2007/149219 is based, given constant symbol time, on a 16 QAM (Quadrature Amplitude Modulation) modulation, with two amplitude states and eight phase states.
However, this solution is complex to transpose to operation at 13.56 MHz, because, on the card side, it is necessary to be able to select both the retromodulation type and the value of the retromodulation load applied to the antenna.
The method described in this document makes it possible to encode a message m of n bits by simultaneously modulating the phase and the amplitude of a carrier frequency. With each n-bit message to be transmitted, a mapper (constellation diagram) associates a point of a complex plane.
However, this constellation diagram, which associates a phase and an amplitude of the carrier frequency, cannot be associated with modulation patterns.
The document WO 03/088499 relates to an encoding method between a reader and a passive transponder.
This solution proposes an encoding scheme in which n phases (n being an integer number) of a subcarrier are used to encode n bits in a time normally provided for 1 bit in a conventional encoding scheme.
Alternatively, this document proposes the use of different subcarriers instead of or in combination with the n phases.
With reference to FIG. 5 of this document, 3 bits are encoded per symbol either with eight subcarrier cycles for the symbols 1 to 4, or with six subcarrier cycles for the symbols 5 to 8.
Assuming an operation with a carrier frequency at 13.56 MHz and symbol time of 8/fSC (subcarriers at 848 kHz and 636 kHz), it can be deduced therefrom that the bit rate achieved is only 318 kbits/s.
Moreover, the document U.S. Pat. No. 6,422,476 describes a data compression/decompression method for minimizing the memory size occupied by a character stream.
The character streams disclosed in this document are only symbols in the sense of this patent application, that is to say elements of an encoding alphabet.
This document does not in any way disclose modulation patterns which have to take account of the physical transmission of the data.